Conventionally, a loop antenna is used in a portable radio communication apparatus, in particular, a mobile telephone. A configuration of the loop antenna is disclosed in, for example, a prior art document of “Institute of Electronics and Communication Engineers of Japan (IECE) editor, “Antenna Engineering Handbook”, pp. 59-63, Ohm-sha Ltd., First Edition, issued on Oct. 30, 1980”. The total length of the loop antenna is normally about one wavelength, a structure of the loop antenna can be approximated to a structure, in which two half wavelength dipole antennas are aligned, based on its current distribution, and the loop antenna operates as a directional antenna having a directivity in a loop axis direction.
When the size of the loop antenna is reduced to have a total length of 0.1 wavelengths or less, a distribution of a current flowing in a loop conducting wire is substantially constant. The loop antenna in this state is referred to as a minute loop antenna. Since the present minute loop antenna is robuster over a noise electric field than a minute dipole antenna and its effective height can be easily calculated, the minute loop antenna is used as an antenna for use in magnetic field measurement.
The present minute loop antenna is widely employed as a small-sized one-turn antenna in the portable radio communication apparatus such as a pager or the like. Since an input resistance of the minute loop antenna is normally quite low, there have been developed a multi-turn minute loop antenna having a multi winding structure so as to remarkably stepwise increase the input resistance. It has been known that the minute loop antenna operates as a magnetic ideal dipole (or a magnetic current antenna) and exhibits a favorable antenna gain characteristic even when a metal plate, a human body or the like is located closely thereto.